This article highlights that engineers at TU Electric’s largest plant tested the flow rate of the water circulating system, which helped them to reduce its pumping power consumption by 9 percent. The plant hired a company to run a calibrated turbine meter through the pipes. The total generating capacity at TU Electric’s plants tops 20 million kilowatts. TU Electric’s Martin Lake plant, in Rusk County, Texas, has three units, each with a capacity of 750 megawatts, which makes it the largest-capacity plant in the utility’s system. The first field measurements at the Martin Lake plant were conducted for TU Electric by Encor–America Technologies 1nc. Encor subcontracted with Flow Simulation Services (FSS) to perform a CFD simulation. CFD involves the solution of the governing equations for fluid flow at thousands of discrete points on a computational grid.
Engineer's at TU Electiric's largest plant wanted to test the flow rate of the water circulating system, so they hired a company to run a calibrated turbine meter through the pipes. What they found was turbulence—enough to knock out the test equipment.
By the time the utility finished investigating and making corrections, it had reduced the plant's pumping power consumption by 9 percent.
TU Electric owns and operates 67 electric generating units running on various fuels at 24 plants. TU Electric is a subsidiary of Texas Utilities Co., a Dallas-based holding company of energy service providers that posted revenues of $7.9 billion in its latest annual report.
The total generating capacity at TU Electric's plants tops 20 million kilowatts. Through leases and other agreements, the company has access to a reserve of more than 2 million kilowatts. TU Electric's Martin Lake plant, in Rusk County, Texas, has three units, each with a capacity of 750 megawatts, which makes it the largest-capacity plant in the utility's system. The three units went into operation in the late 1970s.
The circulating water system at each of the Martin Lake units handles 400,000 to 500,000 gallons a minute. A horizontal 10-foot-diameter inlet pipe connects to a 59-foot-Iong inlet block. Four 7 -foot-diameter vertical outlet pipes are connected to the top of the inlet block. A turn at the top of each outlet pipe returns the flow to a horizontal direction. As a result, the high-momentum watercourse makes two 90-degree turns before entering the heat exchanger tube sheets.
The first field measurements at the Martin Lake plant were conducted for TU Electric by Encor-America Technologies Inc. The tests, besides damaging a rotor on Encor's test meter, confirmed the existence of very turbulent flow conditions, especially within outlet pipe B. These measurements also revealed a somewhat unbalanced flow split among the four condensers fed by the outlet pipes. Encor's tests found that the rotational component of the turbulence was of the same magnitude as the axial component.
Encor subcontracted with Flow Simulation Services (FSS) to perform a CFD simulation. CFD involves the solution of the governing equations for fluid flow at thousands of discrete points on a computational grid. When properly validated, a CFD analysis allows engineers to determine the direction and speed of flow at any point in the model. The simulation provides more information than point sensors in a scale model or an actual circulating water system.
Refinements in CFD software and the escalation of power in personal computers have made it possible to perform analyses at a small fraction of the cost of building a physical model. Because the geometry of the CFD model can be changed quickly on the computer and reanalyzed to explore different options in design or operating conditions, engineers are able to evaluate a wide range of alternative designs at minimal expense. Each test provides useful feedback and understanding that usually helps the analyst move quickly to a solution.
FSS selected the FLOW-3D CFD software from Flow Science Inc. of Los Alamos, N .M., to perform the analysis. The advantage of this software package in circulating water applications is its ability to quickly and easily set up the desired flow geometry and accurately model large-scale turbulence.
The final CFD model developed by FSS was three-dimensional and included the inlet pipe, inlet block, condenser feed pipes, the four waterboxes (where the water turns a second time) , and the tube sheets. The model of the block included three floor-to-ceiling pylons between the condenser feed pipes and smaller vortex breakers located below each pipe. The model consisted of more than 350,000 computational cells and included flow turbulence effects. The model took 30 hours to solve on a personal computer.
The results of the initial simulation showed high-velocity vectors around each of the pylons, indicating that they were strongly influencing the flow in the block. The simulation also showed the formation of large-scale vortices below each inlet pipe that persisted up into the condenser feed pipes. The cyclonic pattern was so pronounced that in many areas the horizontal velocity of the vortex was stronger than the vertical flow of water up the pipe toward the condenser. Vertical sections in the waterbox area revealed a dramatic flow transition at about the mid-height of the intake pipe. A vertical vortex formed at the perpendicular turn into the water box.
The presence of high-flow oscillations in the immediate vicinity of the pylons prompted FSS analysts to consider altering the three pylons in the inlet block. They first tried a model with holes in the pylons, but found that this reduced but did not elin1inate the cyclonic flow.
The next FSS model removed the pylons altogether and found that this significantly reduced the horizontal vortex. However, TU Electric engineers believed the benefit of removing the pylons would not justify the cost of breaking them up and getting them out of the concrete inlet block. Structural calculations indicated that removing several feet of the pylons would significantly improve the flow of water.
The analysts created a new model that reflected the removal of parts of the pylons and all the vortex breakers. The results showed a substantial reduction in turbulence in the inlet block. The model also showed that upward flow velocity within the block took a relatively uniform pattern below each of the four condenser intake pipes.
TU Electric engineers made these structural modifications at the Martin Lake Unit 2 during a shutdown. On startup, it was immediately apparent that vibration in the inlet block was substantially reduced. To confirm the flow balance, Encor was again asked to measure the flow into each condenser. The measurement process confirmed that flow turbulence had been virtually eliminated, but showed that the flow was unbalanced by a higher flow through condenser B. That is when the model was expanded to include the water boxes.
In a closer study of CFD results, a horizontal cross-section of intake pipe B revealed a downward velocity of 74 centimeters a second in a small sector, not evident in the original model. After factoring this reversal into Encor's flow algorithm, the measured flows proved to be well balanced and to match the analysis results.
The combination of CFD and empirical data led to the solutions that eliminated turbulence and vibration in the inlet block and substantially reduced the amount of pumping power required to feed the condensers.
After making the design changes indicated by the analysis, TU Electric reduced the power it needed to run the entire circulating water system, from the inlet pipe through the condensers, by about 9 percent. That translates into a reduction of about 410 kilowatts in the power draw required to pump water through the system